1. Field of the Invention
The present invention relates to a display device in which a thin film transistor is used at least in a pixel portion and a manufacturing method thereof.
2. Description of the Related Art
In recent years, technology that is used to form a thin film transistor using a semiconductor thin film (with a thickness of from several nanometers to several hundreds of nanometers, approximately) formed over a substrate that includes an insulating surface has been attracting attention. A thin film transistor is broadly applied to electronic devices such as an IC and an electrooptic device, and particularly, a thin film transistor as a switching element of an image display device is expected to be developed quickly.
As a switching element of an image display device, a thin film transistor using amorphous semiconductor film, a thin film transistor using polycrystalline semiconductor film, or the like is used.
As a method for manufacturing a thin film transistor, a method in which a stacked structure is formed by a photolithography process using a large number of light-exposure masks (also referred to as photo masks) is employed. However, it is important for reduction in manufacturing cost or improvement of a yield to reduce the number of steps, and thus technology for reducing the number of photolithography steps, that is, reducing the number of light-exposure masks is developed (for example, see Patent document 1: Japanese Published Patent Application No. 2001-313397 and Patent Document 2: Japanese Published Patent Application No. 2007-133371).